Deploy starts by hooking into the deploy:setupProviderConfiguration lifecycle. It fetches the basic CloudFormation template from lib/templates and replaces the necessary names and definitions with the one it gets from the serverless.yml file.

Next up it deploys the CloudFormation template (which only includes the Serverless S3 deployment bucket) to AWS.

In the end it hooks into deploy:deploy lifecycle to update the previously created stack.

The resources section of the serverless.yml file is parsed and merged into the CloudFormation template. This makes sure that custom resources the user has defined inside the serverless.yml file are added correctly.

Note: Empty, but defined Resources or Outputs sections are set to an empty object before being merged.

Next up it removes old service .zip files in the services S3 bucket. After that it uploads the services .zip file (which is available via serverless.service.package.artifact) to the S3 bucket (which is defined in the core CloudFormation template). Furthermore it updates the stack with all the Resources which are defined in serverless.service.resources.Resources (this also includes the custom provider resources).

The stack status is checked every 5 seconds with the help of the CloudFormation API. It will return a success message if the stack status is CREATE_COMPLETE or UPDATE_COMPLETE (depends if you deploy your service for the first time or redeploy it after making some changes).